1. Field of the Invention
The present invention relates to a semiconductor device including a storage element and a method of manufacturing the semiconductor device. The present invention particularly relates to a semiconductor device using a fuse element as the storage element.
2. Description of the Related Art
In a society as in the present day society using many electronic appliances, a variety of data are generated or used, and storage devices (hereinafter referred to as “memory”) are necessary for storing these data. The variety of memories produced or used currently all have advantages and disadvantages, and they are used according to use or the type of data to be handled.
The memories can be roughly classified into volatile memories and non-volatile memories. A volatile memory is a memory whose stored content is lost if power is not supplied. A non-volatile memory is a memory whose stored content is retained even without a supply of power. A DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory), or the like is given as the volatile memory. Although use of the volatile memory is very limited since the stored content is lost when power supply is turned off, the volatile memory is used for a cash memory of a computer or the like because the volatile memory has short access time. Further, although increase in capacity is easy with the DRAM since its memory cell is small, a controlling method is complex and power consumption is high. Furthermore, although a memory cell of the SRAM includes a CMOS and a manufacturing process and control method are easy, it is not suitable for an increase in capacity because six transistors are necessary in one memory cell.
The non-volatile memories whose storage contents are retained without a supply of power can be roughly classified into three categories, which are a rewritable type, a write-once type, and a mask-ROM (Read Only Memory). In the rewritable type, stored content can be rewritten within a limited number of times it can be rewritten. In the write-once type, a user of the memory can write in data only once. In the mask ROM, data content is set during manufacturing and the data content cannot be rewritten.
As the re-writable nonvolatile memory, an EPROM, flash memory, ferroelectric memory, or the like is given. Writing operation of the EPROM is easy and unit price is relatively low, but a dedicated programming device and erasing device are necessary for writing and erasing. In the flash memory or ferroelectric memory, rewriting can be done over a substrate that is used, and it has short access time as well as low power consumption; however, unit price per bit is high because a step of building-in a floating gate or a ferroelectric layer during manufacturing is necessary.
A memory cell of a write-once nonvolatile memory includes a fuse or anti-fuse, a cross pointer diode, an OLED (Organic. Light Emitting Diode), a bistable liquid crystal element, or another device whose state changes by heat or light. Usually, data is stored by each memory cell in the memory being in one of two states (a first state and a second state). In the write-once storage device, all memory cells are manufactured to be in one state (the first state) during manufacturing, then selected memory cells are changed to the other state (the second state) by a writing process. This change from the first state to the second state is irreversible, and a memory cell that is changed once cannot be returned to the original state.
Note that an IC tag can be given as an example of a semiconductor device in which a memory and another functional circuit is built over a substrate. A memory included in an IC tag is an SRAM, a mask ROM, a flash memory, a ferroelectric memory, or the like. Since data content is set during manufacturing memories in the mask ROM, a user of the IC tag cannot write in data. Further, since one memory is necessary for one data, when a memory of different data content is necessary, a photomask corresponding to each type of data becomes necessary. Accordingly, a mask ROM is not practical in terms of cost.
A manufacturing process of the aforementioned write-once memory often differs with those of a central processing unit (hereinafter referred to as “CPU”), an arithmetic function circuit, a rectifying circuit, a control circuit, and the like (these circuits are hereinafter referred to as “other functional circuits” to differentiate from the write-once memory). Accordingly, even when the memory and TFTs forming the foregoing circuits are to be manufactured over the same substrate, it is necessary to form them using different materials in different manufacturing steps.
Note that for the flash memory or ferroelectric memory, a special step to build-in the memory, such as a step of building-in a floating gate or a ferroelectric layer in a gate insulating layer as mentioned above, is necessary. On the other hand, circuits other than the memory fanned in an IC tag can be built-in within the manufacturing process of CMOS. That is, the other circuits can be manufactured using a manufacturing technique of a TFT (Thin Film Transistor). Therefore, if a useful memory can be manufactured by the TFT manufacturing technique, a semiconductor device in which a memory and other functional circuits are mounted over the same substrate can be manufactured.
That which is called an IC tag, ID chip, or the like as mentioned above can be manufactured by a TFT manufacturing process. In the ID chip, necessary information is stored in a storage element in the IC chip, and the information inside is read using non-contact means, or in general, wireless communication means. By practical application of such an ID chip, commodity distribution and the like are expected to be simplified, improved in efficiency, reduced in cost, and improved in security.
The ID chip includes a memory holding portion for identifying individual information. The ID chip is differentiated from other ID chips according to the stored individual information. With this, identification of a product or management of stock is possible. An example of individual authentication using an ID chip is described with reference to FIG. 4. FIG. 4 is a conceptual diagram of individual authentication by which individual information of a bag is read without contact. An ID chip 401 storing specific individual information is provided to a bag 404 by being attached to the outside of the bag 404. An electromagnetic wave is transmitted to this ID chip 401 from an antenna unit 402 of a reader/writer 403. When the ID chip 401 receives the transmitted electromagnetic wave, the ID chip 401 sends individual information of the ID chip 401 to the antenna unit 402. The antenna unit 402 sends the received individual information to the reader/writer 403, and the reader/writer 403 differentiates the individual information. In this manner, information of the bag 404 is obtained by the reader/writer 403. By using a system such as that shown in FIG. 4, distribution management, tabulation, elimination of counterfeited items, and the like become easy.
As one of such techniques by which individual information is stored in individual ID chips, there is a fuse memory element (a storage element of a nonvolatile memory using a fuse element). A fuse memory element is a storage element that stores information by selectively applying high voltage to a memory cell to insulate the selected place or make it be in a state of high resistance. A conventional fuse memory element is insulated by selectively breaking a junction or by melting (For example, Patent Document 1: Japanese Published Patent Application No. 2005-251183).
On the other hand, there is an anti-fuse element as an element similar to the fuse element. The anti-fuse element has high resistance in the first state, but by applying voltage, it transitions to the second state with low resistance (for example, Patent Document 2: Japanese Published Patent Application No. Hei 5-136269).
When a junction is broken or melting is carried out as done with a conventional fuse memory element, a region with sufficient area is necessary so that a broken junction portion or a melted portion can be insulating for sure, which becomes a hindrance in size reduction. It is particularly unsuited for an IC tag or the like in which miniaturization of a pattern wiring and size reduction of a device is demanded. Also, since a structure itself changes in shape by the break of the junction and by melting, another wiring is affected in no small measure within an element.
Further, there are still problems with the conventional semiconductor device such as the following. One is that when a mask ROM is used in a memory circuit, writing cannot be carried out except for during chip manufacturing. Therefore, an ID chip to which data can be written other than during chip manufacturing is in demand. Also, when an EPROM (Erasable Programmable Read Only Memory) typified by an EEPROM (Electronically Erasable and Programmable Read Only Memory) is used for a memory circuit, although a user can freely rewrite content, counterfeiting is also possible since a person other then the right user can rewrite information for authentication that should not be rewritten. This is a serious fatal flaw from a perspective of ID tag security. Accordingly, an ID chip that can only be written once for preventing such counterfeiting is in demand. As such memory that can only be written once, which maintains storage by insulating a fuse portion or making the fuse portion to have high resistance, there is a PROM (Programmable Read Only Memory).